The present disclosure relates to image forming apparatuses and particularly relates to a technique for keeping the temperature of a heat roller of a fixing section uniform across every surface region thereof in a direction of a rotary axis thereof.
A typical image forming apparatus includes a fixing unit for fixing an unfixed toner on a recording paper sheet using a heat roller and a pressure roller (the heat roller and the pressure roller may also be hereinafter collectively referred to as a fixing roller pair). In the fixing unit, when a recording paper sheet passes through a fixing nip position which is an engagement position between the heat roller and the pressure roller, the fixing roller pair causes the phenomenon that a central region thereof in the direction of the rotary axis which comes into contact with the recording paper sheet becomes lower in temperature than end regions thereof out of contact with the recording paper sheet, resulting in a temperature difference between the regions of the fixing roller pair in the direction of the rotary axis. If the next image formation and fixing are performed as the temperature difference remains, the formed image may cause an undesirable color shading in the direction of the rotary axis. Therefore, to eliminate this effect on the next image formation, the fixing unit, after the end of the current image formation, performs a post-drive allowing the fixing roller pair to rotate in the absence of any recording paper sheet before the passage of a next recording paper sheet to reduce the temperature difference between the regions of the fixing roller pair in the direction of the rotary axis and thus stabilize the temperature of the fixing roller pair.
In a fixing unit of the type in which a central region and an end region of a heat roller are provided with their respective temperature sensors, the above post-drive is stopped at the point of time when the temperatures of the central and end regions of the heat roller detected by their respective temperature sensors reach the same temperature. On the other hand, in a fixing unit of the type in which only an end region of the heat roller is provided with a temperature sensor, such as that in an image forming apparatus produced at low cost, it is impossible to control the post-drive based on the determination of whether the central and end regions of the heat roller reach the same temperature. Therefore, the post-drive is performed only for a predetermined period of time after the end of the current image formation.
In an exemplary image forming apparatus intended to eliminate the temperature difference between the regions of the heat roller in the direction of the rotary axis, the on/off ratios of two heaters provided inside the heat roller are determined according to the fixing conditions and the operation of the two heaters is controlled based on these ratios to eliminate the temperature difference between the regions of the heat roller in the direction of the rotary axis.